Fortunately, it does not have to be the case that your immune system is weaker. Oral galvanism is certainly an issue too, and it makes a huge difference in how fast the fillings corrode. Some people just have higher currents in their mouth. Also, if you are often close to stronger EMF fields (monitor, cellular phone) and wear headphones etc. the fillings corrode even much faster. I know this, as my salivation always increases when I sit in front of a monitor, and I can't tolerate wearing headphones for a longer time.
So the EMFs seem to work both ways, getting mercury poisoned and taking some of the stuff out again.